Method and apparatus for treating pulp

ABSTRACT

The present invention relates to a method of and apparatus for treating pulp. More precisely, the present invention relates to a method and apparatus for treating pulp with oxygen. Especially preferably the method and apparatus according to the invention relate to oxygen delignification of chemical pulp, i.e. so-called oxygen bleaching.  
     A characterizing feature of the method according to the invention of treating chemical pulp in an oxygen bleaching stage where the pulp is bleached in at least one pressurized reactor in the presence of alkali is that the whole oxygen stage is effected pressurized and that a bigger amount of alkali than conventional is fed in the beginning of the oxygen stage and that the whole alkali dose is fed in the beginning of the oxygen stage.

[0001] The present invention relates to a method and an apparatus for treating pulp. More precisely, the present invention relates to a method of and an apparatus for treating medium consistency (8-18%) pulp with oxygen. Especially preferably the method and apparatus according to the invention are applicable to oxygen delignification i.e. so-called oxygen bleaching of chemical pulp.

[0002] The oxygen delignification has recently become the most popular method of delignifying pulp after the cook. By means of oxygen delignification it has been possible to significantly reduce the chemical consumption of the D₀ stage used for the same purpose. Most suppliers nowadays offer a two-step oxygen stage that has made a significant market penetration. A two-step oxygen stage means a delignification stage that is performed in two reactors, i.e. towers, without intermediate washing between the reactors.

[0003] One of the earliest publications relating to the modern two-step oxygen stage is U.S. Pat. No. 5,217,575. According to said publication, the oxygen stage has been divided to be performed in two towers in such a way that each of the towers is a so-called up-flow tower and the reaction temperature of the first tower is between 70-90° C. and the reaction temperature of the other tower is between 90-125° C., whereby the temperature difference between the towers is between 20-40° C. Further, the publication teaches that both oxygen and alkali may be introduced to the pulp via a mixer located between the towers.

[0004] U.S. Pat. No. 5,034,095 deals mainly with the construction of the reaction tower used in the oxygen stage, but also describes how the oxygen stage may be effected by a two-step method. According to the publication, both alkali and oxygen are introduced into the pulp prior to the first tower and the bleaching in the first tower is effected under a pressure of 0-7 bars and a temperature of 70-140° C. The same reaction parameters are used also in the second tower. The publication also allows for additional introduction of both alkali and oxygen into the pulp between the towers.

[0005] WO-A-97/20983 deals with a two-step oxygen stage in which the treatments are carried out at a temperature of at least 75° C. and stabilizing the pH by introducing alkali into both steps. The duration of the treatment in the first step is about 5 minutes and in the second step about 55 minutes.

[0006] WO-A-97/27358 handles a two-step oxygen stage in which the oxygen bleaching is carried out at a temperature of 75-115° C. and the pH is kept constant by adding alkali between the steps. According to the publication, hydrogen peroxide is further fed into the second step as additional chemical.

[0007] U.S. Pat. No. 5,460,696 deals with an oxygen stage that has been divided into three steps, each of which is pressurized and run at a high temperature (about 115° C.). A characterizing feature of the process according to said publication is the aim to keep the alkali concentration in the pulp constant throughout the whole process by adding alkali into each of the towers separately.

[0008] SE patent 505 141 handles in principle a process according to U.S. Pat. No. 5,217,575. The only major difference is that the temperature difference between the towers is less than 20° C. Further the publication teaches how the whole alkali charge is fed into the first tower and how a higher pressure is used in the first tower than in the second tower.

[0009] In practice, all the above mentioned publications are based on the commonly accepted fact that while a high alkali concentration improves the delignification degree, at the same time it allows the alkali to react with the carbohydrate chains of the fibers, which decreases the viscosity. For this reason, the aim is to keep the alkali concentration constant during the whole oxygen bleaching process (compare to U.S. Pat. No. 5,460,696) or right from the beginning of the process under a certain limit in order to maintain the strength of the pulp. Said publications also teach how the oxygen bleaching should in practice be performed either at a temperature of at least 100° C. or at least so that part of the bleaching stages are performed at said temperature.

[0010] Iribarne, J. and Schroeder, L. R. have in their article High-pressure oxygen delignification of kraft pulp, Part 1: kinetics. Tappi Journal, October 1997, Vol 80, No:10. 241-250 dealt with numerical simulation of the oxygen stage. One of the items under consideration in this article is e.g. what is the best way to maintain the strength of the pulp measured by viscosity during the oxygen stage. E.g. in case of a two-step oxygen stage the article recommends to keep the temperature in both steps relatively low, to make the first step short in time and to perform the first step both under high pressure in order to keep the solubility of the oxygen high and at a high alkali concentration, and to perform the second step at atmospheric pressure. The publication even finds it detrimental to perform the second step pressurized. This kind of presumption is not surprising per se, as prior art knows several processes in which the pressure is decreased between the oxygen steps.

[0011] A more thorough analysis of the kinematic model presented in the article leads to a common statement that the selectivity of the delignification reaction is maximized i.e. the higher the partial pressure of the oxygen (that is, the solubility of the oxygen) is and the higher the alkali concentration, the higher the viscosity of the pulp may be maintained. According to the publication, the oxygen stage should be divided into two steps so that the whole oxygen charge and all of the alkali should be fed in prior to the first reactor and the oxygen concentration should be decreased as much as possible e.g. by decreasing the pressure in the reactor to atmospheric pressure when entering the second step, whereby it would be possible to avoid disadvantages which according to the publication are caused by a great oxygen concentration. According to the article, alkali concentration in the second step has no significance. As to the temperature, it is stated that both steps should be run at as low a temperature as possible in view of the desired delignification degree.

[0012] On the basis of our experiments we have, however, noticed that partly said article does not correspond to reality, even though it in many respects does give good advise to the developers of the oxygen stage.

[0013] One of the characteristic features of the bleaching method accoring to our invention is that the whole oxygen stage is performed pressurized, the amount of alkali introduced into the oxygen stage (kg/bdt as NaOH) is at least 2.0 times the desired kappa reduction and the whole alkali charge is fed in the beginning ot the oxygen stage. Thus, a characteristic feature of the invention is that additional alkali is introduced into the pulp entering the bleaching stage so that the alkali charge is essentially (at least about 25%) bigger than the conventionally used charge. “Conventional charge” is used to refer to the amount of alkali needed in order to increase the pH of the pulp to a desired level for oxygen delignification. In other words, in the method according to the invention, a certain amount of excess alkali is fed into the oxygen stage.

[0014] One of the characteristic features of a preferred embodiment of the bleaching method according to our invention is that in case of a two-step oxygen stage, the second step is run not at atmospheric pressure but preferably at at least the same pressure as the first step.

[0015] Other characteristic features of the method according to the invention are disclosed in the appended claims.

[0016] In the following, the method according to the invention is disclosed in more detail with reference to the following figures, of which

[0017]FIG. 1 illustrates a two-step oxygen bleaching stage according to a preferred embodiment of the invention and

[0018]FIG. 2 illustrates schematically a combination of a digester, brown stock washing and oxygen bleaching according to a second preferred embodiment of the invention.

[0019] First, some background information about the developing of the oxygen stage according to our invention. In developing a new type of oxygen stage, our starting point was the prior art truth that by keeping the alkali concentration high after the O₂ stage, the viscosity of the pulp may also be kept at a good level. However, after this followed a deviation from the teachings of prior art, i.e. from keeping the alkali concentration uniform throughout the bleaching, so that additional alkali was added into the O₂ stage prior to the first reactor. In this way, the alkali concentration was raised higher than in previous prior art processes. When practicing the method according to our invention, the alkali concentration is further raised by means of washings prior to and after the oxygen stage. From the wash following the oxygen stage the alkali that has not been consumed in the reactions in the oxygen stage is separated into the filtrate and returned into the oxygen stage feed when the washing filtrate from the washer/s following the oxygen stage is being led as washing liquid to the washer prior to the oxygen stage. Thus, alkali is kind of concentrated in the oxygen stage.

[0020] Maintaining high alkali concentration in the oxygen stage is facilitated also if cook-originating organic material has been washed off from the pulp as thoroughly as possible already before the oxygen stage. Cook-originating organic material present in the pulp consumes alkali thus decreasing the alkali concentration of the pulp and having a decreasing effect on the viscosity after the oxygen stage. Accordingly, a decrease in the alkali concentration, not depending on the cause, has the consequence that in order to obtain the same kappa reduction in the same reaction time, the temperature should be increased. And, as already stated, increasing the temperature results in further viscosity decrease after the O₂ stage.

[0021] The starting point in the present invention is that the objective of the oxygen stage is a certain kappa reduction and that the reaction conditions are optimized in order to obtain optimum viscosity. In a way, the starting point of this invention as such differs from the conventional approach always aiming at the greatest possible kappa reduction with acceptable viscosity loss. By law of reaction kinetics, when additional alkali is fed into the process according to the present invention, the temperatures prevailing in the reactors have to be decreased. As already stated, lower temperature as such facilitates the maintaining of viscosity in the oxygen stage. Thus, additional alkali results in two effects protecting the viscosity, i.e. the strength of the pulp. Firstly, the alkali concentration is kept high, which as such has an advantageous effect in view of maintaining the viscosity of the pulp. Secondly, a higher alkali concentration allows for a lower temperature to obtain the same kappa reduction. In the method according to the invention, the pressure of heating steam possibly required is less than 5 bar (abs.), while prior art oxygen stage processes require medium pressure steam.

[0022] High alkali concentration and low temperature lead to a situation, where all of the alkali is not consumed in the oxygen stage, but some of it is passed to the washing following the oxygen stage. Because the amount of this alkali is of significance when determining the total alkali charge, this alkali amount has to be taken into account as a key factor in order to perform oxygen delignification of the pulp with a uniform kappa reduction. Conventionally, the alkali amount has been so small that at the end of the oxygen stage there has been no alkali left. Now the conditions during the whole oxygen stage are such that there is enough alkali for the reactions in the oxygen stage. Thus, the charge of alkali in the oxygen stage in this case is not the same as the introduced amount of alkali, but the alkali charge is formed of two factors; the amount of alkali introduced and the amount of alkali returned via washing.

[0023] In order to determine the required alkali charge, the feedback for the alkali being transferred via washing to the oxygen stage has to be arranged so that there is a sufficient indication of the properties of washer filtrate. This may be performed e.g. in such a way that the pH of the filtrate/solution is measured and utilizing the measurement results the amount of residual alkali is calculated. Various automated liquor analyzers may also be used for this purpose, or the properties of the filtrate may be determined in a laboratory. Further, the same properties have to be determined concerning the pulp prior to the oxygen stage to ensure an adequately precise dosing. Thus, it has been possible to determine the alkali charge of the oxygen stage so that both the amount of alkali passing via the filtrates and the amount of alkali dosed directly with the pulp are known. Additionally, the consistency of the pulp passing to the oxygen reactor has to be determined in order to determine the alkali charge on the basis of the properties of the solution.

[0024] As the temperature of the oxygen stage accelerates the reaction, the temperature has to be regulated aiming at keeping the velocity of the reaction constant. This is effected so that when the amount of alkali dosed and circulated via washing is known, and thus the alkali charge into the oxygen stage determined, the target temperature of the oxygen stage is calculated utilizing a kinetic model in order to obtain the desired reaction velocity. The reaction velocity in this stage is dependent on said factors: the alkali charge, the temperature, to some extent the dosing of oxygen and process-initiating factors, such as the washing efficiency, washing retention and accordingly the amount of cook-originating black liquor in the process. Because all these factors are process-specific, the mathematical model has to be corrected for each specific process.

[0025] In order to correct the model precisely enough, one has to be able to monitor the progress of the reaction during the process. The easiest way to do this is kappa measurement, which by means is not the only applicable method. Other possible methods are e.g. monitoring the following variables: the velocity of alkali consumption, the increase in the dry solids content, the increase in the amount of lignin, or some other factor considered suitable. By a combination of these factors it is possible to correct the model so that the model for each specific case is capable of calculating the suitable temperature for the specific process in question so that the alkali concentration around the fibers remains desirable and that the reaction progresses in desired time to the desired kappanumber. The required feedback may be effected e.g. using a temperature profile. In such a case, the reaction temperature required for a specific reaction velocity is calculated for the model, and the progress of the reaction is checked often enough by means of the temperature profile of the reactor, either on the surface of the reactor or inside it.

[0026] It has also been noticed that greater solubility of oxygen has a protecting effect on viscosity. According to the earlier mentioned publication of Iribarne, only at the beginning of the oxygen stage. The solubility of oxygen is the higher, the higher is the partial pressure of the oxygen, and the lower the temperature. On the other hand, it has to be admitted that a higher alkali content at a constant temperature decreases the solubility of oxygen, although the effect thereof is in practice almost non-existing. Our experiments show that oxygen bleaching may be run to the end at a high oxygen concentration without disadvantages claimed by Iribarne in his article.

[0027] Further, according to our experiments, a two-step embodiment of the oxygen stage is a good solution, provided that the pressure of the second step may be increased. This may be effected e.g. by means of a pump located between the steps. A second possibility is to use a pressure-raising discharger at the top of the first reactor. If it is not possible to raise the pressure between the reactors, it is preferable to run the oxygen stage in one step. In that case the only thing in favor of a two-step run of the oxygen stage is the gradual accumulation of oxygen to form bigger bubbles in the pulp as the reaction progresses, whereby the solubility of the oxygen decreases accordingly. When feeding the pulp into the second tower, the device used for feeding, either a discharger and/or pump and/or mixer breaks the gas bubbles and distributes the oxygen evenly into the pulp in form of small bubbles, thus restoring the solubility of the oxygen.

[0028] Retention times applied in reactors in a two-step oxygen stage are dependent on the feeding pressures of the first and second step. If the feeding pressures are equal, the reactors have to be of the same size. The higher the feeding pressure of the first step compared to the feeding pressure of the second step, the longer retention time is needed in the first reactor. The natural reason for this is to make it possible to utilize the high pressure of the reactor to a maximum degree.

[0029]FIG. 1 illustrates schematically an oxygen bleaching stage 10 according to a preferred embodiment of the invention. The oxygen stage 10 of the figure has two steps, i.e. it comprises two reactors 12 and 14. The first reactor 12 is preceded by at least one, possibly several, mixing device/s, preferably a pump 16 and an mixer 18, which device/s is/are used for mixing at least alkali and oxygen into the pulp coming from a previous process stage, preferably from brown stock washing. A characterizing feature of the process according to the present invention is that all alkali required in the oxygen stage, which alkali may be oxidized white liquor or some other alkali suitable for the purpose, is mixed in the pulp prior to the first reactor 12. The oxygen, in turn, may be mixed either completely in the pulp flow going to the first reactor 12 or partly in the pulp going to the second reactor 14.

[0030] Even if all the oxygen was fed into the first reactor 12, it is nevertheless preferable to arrange between the reactors 12 and 14 a mixer 18, by means of which the pulp flow being transferred from one reactor into the other is efficiently mixed. The purpose of the mixing is to break the gas bubbles accumulated in the pulp forming major bubbles into smaller bubbles, whereby material transfer from the gas bubbles into the surrounding fiber suspension is essentially improved.

[0031] In addition to the mixer 18 arranged between the reactors, the gas bubbles may be broken also in addition to the mixer 18 or alternatively by means of a discharger 20 arranged at the top of the first reactor 12. By means of said discharger 20, it is also possible to raise the pressure of the pulp, if desired, when transferring the pulp into the second reactor 14. Naturally, the pressure increase may also be effected by arranging between the towers 12 and 14 a suitable pump, being well applicable for breaking the gas bubbles.

[0032] It is preferable to arrange at the top of the second reactor 14 a special gas-separator or a gas-separating discharger 22, by means of which gas is separated from the pulp going from the oxygen stage into a following process stage, most usual washing, in order to decrease foaming problems. This is especially preferable in the process according to the invention, in which the pulp leaving the oxygen stage has a high alkali content.

[0033] This kind of two-step bleaching stage is run at a lowest possible temperature. In practice, the most common result is that the aim is to keep both reactors practically at the same temperature. The temperature ranges are 75-105° C., preferably 80-95° C., most preferably 80-85° C. Naturally, the same temperature range applies to a single reactor of a one-step oxygen stage. The pressure in the first reactor is in the range of 8-20 bar, preferably 8-12 bar. In the second reactor, the pressure is in the range of 5-20 bar, preferably 8-12 bar. The applied treating times in the first reactor are in the range of 5-45 minutes, preferably 10-30 minutes, and in the second reactor 20-180 minutes, preferably 30-50 minutes.

[0034] The amount of alkali to be fed into the oxygen stage is dependent on several factors. These include e.g. the desired kappa reduction, the wood species, the purity of the pulp etc. The amount of introduced alkali is between 10-100 kg/adt, preferably 30-70 kg/adt, most preferably 40-60 kg/adt. An essential issue in the feed of the alkali is that the introduced amount is essentially greater than conventionally. The above mentioned article of Iribarne discloses the required alkali charge in proportion to the desired kappa reduction in form of coefficient 1.33, which means that if the desired kappa reduction is 10, the amount of alkali to be introduced is 13.3 kg/bdt as NaOH. Some other sources mention a somewhat higher coefficient, so that the starting point here may be that a coefficient in the range of 1.5 represents the upper limits of a conventional charge and a coefficient in the range of 2.0 or more, represents an alkali charge according to the present invention.

[0035]FIG. 2 illustrates a connection of an oxygen stage 10 according to e.g. FIG. 1 to the chemical pulping process. The figure illustrates schematically how the pulp is being discharged from the digester 30 into brown stock treatment represented in the figure only by washers 32 and 34, as other components, such as knotters or screens, have no essential effect on the invention. In practice the number of washers is one or more, depending e.g. on the type they represent. From the washers, the pulp is led into the oxygen stage presented already in connection with FIG. 1.

[0036] The purpose of FIG. 2 is to present the alkali circulation in connection with the oxygen stage. As seen from the figure, all “fresh” alkali required in the oxygen stage is introduced into a mixer 18 or the like mixing device prior to the first reactor. Therefrom the alkali is passed, partly being consumed in the delignification reactions, via the reactors up to the oxygen stage washer 36. Preferably said washer is a DRUMDISPLACER® washer by Andritz-Ahlstrom Corporation, and the alkali-containing washer filtrate, at least a major part of it, obtained therefrom is transferred as washing liquid to a washer 34 preceding the oxygen stage. The washing process effected by said washer has two kinds of results. Firstly, a great amount of residual alkali from washer 36 is passed into the pulp. In practice this means that the alkali content of the pulp entering the oxygen stage 10 is dependent on not only the amount of introduced alkali, but it is further raised by said residual alkali of the oxygen stage. Another result from the wash effected with a high-alkali washing liquid is that a greater amount of alkali is also passed in the filtrate of the washer 34, because part of the washing filtrate from the washer 36 penetrates the pulp cake in the washer 34 so quickly that the alkali thereof is passed also into the filtrate of the washer 34. Most usually said filtrate is taken as washing liquid into a preceding washing device 32, in which exactly the same washing process is repeated.

[0037] In other words, alkali from the washer following the oxygen stage is passed not only back to the oxygen stage, but also countercurrently up to the digester. In our experiments we have noticed e.g. that the amount of alkali passing up to the digester may in the best case be so significant that it reduces the amount of alkali required in the digester. Thus, in practice the whole attention should not be paid to the increased alkali consumption of the oxygen stage, but the digester-oxygen stage should be considered as a whole when determining the operational economy of the process according to the present invention. Another factor clearly improving the economy of the process according to this invention is decreased steam demand. As the oxygen stage is cool, there is essentially no need to heat it, while heating is inevitable in the presently performed hot oxygen stages. Unlike in prior art methods, in the method according to the invention there is no need to use steam having a pressure of more than 5 bar (abs.).

[0038] High alkalinity of pulp is not always considered as a positive thing only. This is the case especially between the digester and the oxygen stage, where the pulp contains great amounts of organic material dissolved into the pulp during the cook. It has been stated that in this kind of conditions high alkalinity combined with a relatively high temperature decrease the viscosity of the pulp. An easy way to avoid this is to arrange the retention time between the digester and the oxygen stage to be as short as possible. The retention time should be, naturally depending also on the alkalinity of the pulp, between 0.5-60 minutes, preferably 1-45 minutes, most preferably 5-15 minutes. The washer/s used is/are preferably a DRUMDISPLACER® washer of Andritz-Ahlstrom Corporation, which washer has a superior washing efficiency and a short pulp retention. Besides, as a whole it is more preferable to use a drum-type washer than a press-type washer, because a drum washer does not deteriorate the fibers as easily as a press mechanically pressing the fibers.

AN EXAMPLE

[0039] The kinetics of the two-step oxygen stage according to our invention has been simulated based on the following starting points. In all cases the kappanumber of the pulp prior to the oxygen stage, i.e. so-called initial kappa is 29.5 and the kappanumber of the pulp after the oxygen stage, i.e. so-called final kappa is 11.8. This corresponds to a kappa reduction of 60%, which is the most common desired value at the present. The TAPPI viscosity of the initial pulp is 26.0 mPas. The diameter of both of the reactors is 3.5 m, consistency in the reactor 11% and production 1000 adt/day. The calculations are based on the assumption that the total retention of the oxygen stage is 60 minutes. NaOH in the tables means the total amount of alkali in the oxygen stage feed, that is, including introduced alkali (oxidized white liquor) and alkali returned via washing. A capital T refers to temperature (unit ° C.) and a non-capital t refers to the retention in the reactor (unit min). Subindex refers to the ordinal number of the reactor (1=the first reactor, 2=the second reactor). Intermediate kappa is the kappanumber after the first reactor and visco is the TAPPI-viscosity after the second reactor.

[0040] From the selectivity equation disclosed in the above-mentioned article of Iribarne, it is possible to make direct conclusions about methods allowing for maintaining the viscosity of the post-oxygen stage at a high level. In practice this means maximizing the right-hand clause of the formula. Thus, according to the equation, the treatment temperature shall be low, the alkali concentration in the beginning of the oxygen stage shall be as high as possible and the solubility of oxygen should also be high. The appended simulation results based on our own calculations also support these observations. In the right-hand side of the Iribarne formula the latter term has a negative effect on viscosity. On the other hand, the coefficient of said term is very small, so that in practice it has no effect.

[0041] The article of Iribarne states that the viscosity after the oxygen stage preferably should not decrease under 15 mPas. If it is so, the amount of alkali entering the oxygen stage should be 55-70 kg/bdt, as shown in the following simulation.

[0042] The appended tables 1 and 4 are in principle the one and same cases in an optimal situation. That means the system has an unlimited number of optimum points. In this case the presumption is that there is no pressure loss between the oxygen stages. In case of pressure loss, the first reactor shall be bigger than the second one, as staged in the above. And, if the pressure is raised between the reactors, the first reactor should be the smaller one. How much smaller depends on how much the pressure is raised between the reactors. If it is noticed that alkali is consumed after the oxygen stage, the amount of alkali fed into the oxygen stage shall be increased. Introduced alkali is understood to be both new alkali and the non-reacted alkali being returned from the oxygen stage wash and possible also residual alkali passing from the cook. If the residual alkali is returned in its “virginal” form into the oxygen stage, the amount of alkali to be added is 20-50% bigger than the normal charge. Otherwise about 100% or even more.

[0043] The model used in our simulation does not take into account the bubble size of the non-dissolved oxygen. Having two steps may contribute to keeping the bubble size small. Neither does the model take into account the dissolved organic load entering the oxygen stage. One way of taking the organic load into account would be to assume that the alkali content decreases. This, in its turn, could be compensated for, in order to obtain the same kappa reduction, either by increasing the temperature of by adding alkali.

[0044] The following table 1 presents the simulation results from a two-step O₂ stage. The variables are both the amount of alkali, NaOH, in the feed and the temperature. No pressure change is assumed to take place between the reactors. All alkali is fed into the first step. TABLE 1 NaOH T₁/ t₁/ Intermed. T₂/ t₂/ Viscos./ kg/bdt C Min Kappa C min Tappi 40.0 80.0 30.0 21.81 106.8 30.0 11.13 40.0 82.5 30.0 21.15 105.9 30.0 11.27 40.0 85.0 30.0 20.40 104.7 30.0 11.42 40.0 87.5 30.0 19.62 103.4 30.0 11.57 40.0 90.0 30.0 18.81 101.7 30.0 11.72 40.0 92.5 30.0 17.96 99.66 30.0 11.86 40.0 95.0 30.0 17.11 97.11 30.0 11.96 45.0 80.0 30.0 21.22 102.6 30.0 12.30 45.0 82.5 30.0 20.47 101.4 30.0 12.46 45.0 85.0 30.0 19.69 99.90 30.0 12.62 45.0 87.5 30.0 18.86 98.18 30.0 12.77 45.0 90.0 30.0 17.99 96.06 30.0 12.90 45.0 92.5 30.0 17.11 93.41 30.0 13.00 50.0 80.0 30.0 20.65 98.69 30.0 13.33 50.0 82.5 30.0 19.83 97.27 30.0 13.49 50.0 85.0 30.0 18.99 95.53 30.0 13.65 50.0 87.5 30.0 18.10 93.40 30.0 13.79 50.0 90.0 30.0 17.10 90.72 30.0 13.89 55.0 80.0 30.0 20.11 95.11 30.0 14.25 55.0 82.5 30.0 19.21 92.42 30.0 14.41 55.0 85.0 30.0 18.31 91.34 30.0 14.56 55.0 87.5 30.0 17.39 88.73 30.0 14.66 60.0 80.0 30.0 19.52 91.71 30.0 15.08 60.0 82.5 30.0 18.60 89.73 30.0 15.23 60.0 85.0 30.0 17.68 87.25 30.0 15.34 65.0 80.0 30.0 18.95 88.52 30.0 15.80 65.0 82.5 30.0 18.03 86.20 30.0 15.92 70.0 80.0 30.0 18.46 85.45 30.0 16.43

[0045] Table 1 shows clearly how especially aiming to a constant kappa reduction has an effect on, in the first place, the temperatures of various steps when the amount of alkali introduced in the stage is constant. In other words, when the first step is run at a low temperature, the kappa reduction of the first step is so small that in order to obtain the desired final kappa value, the temperature of the second step must be raised very high. Secondly, it is noticed how strong an effect the adding of alkali has both on the intermediate kappa and on the final kappa. For comparison it may be stated that with an alkali amount of 40 kg/bdt and a temperature in the first step of 80° C., the temperature in the second step has to be about 107° C. in order to obtain the desired final kappa. In such a case, however, the viscosity decreased to a value of about 11.1 mPas TAPPI, while the desired value according to Iribarne is about 15 mPas. And when alkali was fed in the amount of 70 kg/bdt with the same 80° C. temperature in the first step, the temperature required in the second step for obtaining the desired kappa reduction was only about 85 degrees, and the viscosity was kept at a value of 16.4, i.e. distinctively over the required value of 15 mPas. TABLE 2 NaOH T₁/ t₁/ Intermed. T₂/ t₂/ Viscos./ kg/bdt C Min Kappa C min Tappi 40.0 80.0 30.0 23.54 109.0 30.0 10.44 40.0 82.5 30.0 22.95 108.4 30.0 10.49 40.0 85.0 30.0 22.38 107.9 30.0 10.54 40.0 87.5 30.0 21.76 107.1 30.0 10.58 40.0 90.0 30.0 21.12 106.2 30.0 10.60 40.0 92.5 30.0 20.46 105.2 30.0 10.59 40.0 95.0 30.0 19.78 104.1 30.0 10.58 40.0 97.5 30.0 19.07 102.8 30.0 10.52 40.0 100.0  30.0 18.35 101.3 30.0 10.44 45.0 80.0 30.0 23.06 105.2 30.0 11.48 45.0 82.5 30.0 22.49 104.4 30.0 11.54 45.0 85.0 30.0 21.85 103.7 30.0 11.58 45.0 87.5 30.0 21.20 102.8 30.0 11.61 45.0 90.0 30.0 20.51 101.7 30.0 11.61 45.0 92.5 30.0 19.80 100.5 30.0 11.89 45.0 95.0 30.0 19.06 99.07 30.0 11.54 50.0 80.0 30.0 22.65 101.8 30.0 12.40 50.0 82.5 30.0 22.02 101.0 30.0 12.45 50.0 85.0 30.0 21.36 100.1 30.0 12.48 50.0 87.5 30.0 20.65 99.03 30.0 12.49 50.0 90.0 30.0 19.94 97.75 30.0 12.48 50.0 92.5 30.0 19.16 96.30 30.0 12.42 55.0 80.0 30.0 22.25 98.70 30.0 13.23 55.0 82.5 30.0 21.57 97.81 30.0 13.27 55.0 85.0 30.0 20.87 96.75 30.0 13.28 55.0 87.5 30.0 20.12 95.53 30.0 13.28 55.0 90.0 30.0 19.38 94.03 30.0 13.23 60.0 80.0 30.0 21.83 95.84 30.0 13.98 60.0 82.5 30.0 21.15 94.79 30.0 14.00 60.0 85.0 30.0 20.43 93.61 30.0 14.00 60.0 87.5 30.0 19.62 92.18 30.0 13.96 60.0 90.0 30.0 18.81 90.49 30.0 13.89 65.0 80.0 30.0 21.24 93.59 30.0 14.61 65.0 82.5 30.0 20.70 92.09 30.0 14.64 65.0 85.0 30.0 19.92 90.69 30.0 14.63 65.0 87.5 30.0 19.09 89.08 30.0 14.56 70.0 80.0 30.0 21.02 90.77 30.0 15.23 70.0 82.5 30.0 20.26 89.46 30.0 15.22 70.0 85.0 30.0 19.45 87.93 30.0 15.17

[0046] Table 2, in its turn, further presents the simulation results of a two-step O₂ stage. The variables are, as in the previous table, both the amount of NaOH, this time, however, in the feed of different steps, and the temperature. No pressure change is assumed to take place between the reactors. Two thirds of the alkali is fed into the first step and one third into the second step.

[0047] The only purpose of this simulation example and the table based thereon is to show that it is preferable to feed the whole amount of alkali in the beginning of each stage. As tables 1 and 2 are comparable, it is easy to directly notice that in the simulation of table 2, the viscosity values are as a whole in the average one mPas lower. The reason for this is also easy to find in the table. To put it differently, the temperature of the second step has had to be further raised compared to the example of table 1, when alkali is fed also into the second step. In practice this means exactly that alkali must be fed into the first step as much as possible in order to effect as much as possible of the kappa reduction in the first step.

[0048] Table 3 presents the simulation results of an optimized two-step O₂ stage with different amounts of NaOH in the feed and with different temperatures. This optimization has two starting points, the first one being the aim to the same kappa reduction, and the second one the aim to keep the temperature as low as possible, i.e. in practice the same in both reactors. Additionally, the starting point was to have a partial oxygen pressure of 10 bar at the bottom of both reactors and to introduce all of the alkali into the first step. TABLE 3 NaOH T₁/ t₁/ Intermed. T₂/ t₂/ Viscos./ kg/bdt C Min Kappa C min Tappi 40.0 94.40 27.92 17.82 94.59 32.08 12.39 45.0 91.30 28.85 17.82 91.59 31.15 13.43 50.0 88.70 28.74 18.00 89.06 31.26 14.31 55.0 86.60 28.97 18.00 86.66 31.03 15.09 60.0 84.52 28.79 18.17 84.71 31.21 15.78 65.0 82.74 29.10 18.10 82.91 30.90 16.39 70.0 81.20 29.27 18.17 81.24 30.37 16.92

[0049] According to table 3, as the alkali charge increases, the temperature decreases and the viscosity increases, even so 55 kg/bdt already results in a value of slightly over 15 mPas.

[0050] Table 4 presents the simulation results of an optimized two-step O₂ stage with different amounts of NaOH in the feed and with different temperatures. In this optimization calculation the process has made still more constant. There is no change of pressure or temperature between the reactors and all of the alkali is fed into the first step. A two-step O₂ stage is in practice reduced to a one-step bleaching stage. TABLE 4 NaOH T₁/ t₁ Viscosity/ kg/bdt C. Min Tappi 40,0 95,96 60,0 11,99 45,0 92,91 60,0 13,01 50,0 90,33 60,0 13,90 55,0 88,06 60,0 14,68 60,0 86,04 60,0 15,37 65,0 84,24 60,0 15,98 70,0 82,63 60,0 16,52

[0051] As seen from the above, a quite new method of treating pulp with oxygen has been developed. The above presented will have to be understood only as an example of many variations of the present invention in the scope thereof, which scope is determined by the appended patent claims only. 

1. Method of treating pulp in an oxygen bleaching stage in which the pulp having a consistency of 8-18% is bleached in at least one pressurized reactor in the presence of alkali, characterized in that the whole oxygen stage is run pressurized and that the amount of alkali fed in the beginning of the oxygen stage (kg/bdt as NaOH) is at least 2.0 times the desired kappa reduction and that the whole alkali charge is fed in the beginning of the oxygen stage.
 2. Method according to claim 1, characterized in that alkali fed in the oxygen stage is considered to comprise both fresh alkali and residual alkali passing into the oxygen stage from various washing stages.
 3. Method according to claim 1, characterized in that the oxygen stage comprises multiple steps so that it is performed in multiple reactors, each of which reactor is pressurized.
 4. Method according to claim 3, characterized in that the treatment time of the pulp in each reactor is directly comparable to the pressure of the reactor.
 5. Method according to claim 1, characterized in that the oxygen stage is performed at a temperature of less than 95° C. and a pressure of 5-20 bar, while the treatment time varies between 5 and 180 minutes.
 6. Method according to claim 3 and 5, characterized in that the oxygen bleaching is effected in two steps so that in the first step the temperature is 75-95° C., the pressure is 8-20 bar and the treatment time 5-45 minutes and in the second step the temperature is 75-95° C., the pressure 5-20 bar and the treatment time 20-180 minutes.
 7. Method according to claim 1 or 5, characterized in that the steam used in the oxygen stage has a pressure of less than 5 bar (abs.).
 8. Method according to claim 3 or 6, characterized in that oxygen is fed into the first step only.
 9. Method according to claim 3 or 6, characterized in that oxygen is fed into the pulp between the steps.
 10. Method according to claim 8 or 9, characterized in that the pulp is mixed between the steps in order to break gas bubbles and to ensure a better material transfer between the fibers of the pulp and the gas.
 11. Method according to claim 1, characterized in that the control of the oxygen stage aims at a desired kappa reduction.
 12. Method according to claim 11, characterized in that the required alkali charge is determined on the basis of said desired kappa reduction.
 13. Method according to claims 11 and 12, characterized in that based on at least said kappa reduction and alkali charge and utilizing the treatment time of the oxygen stage, the lowest possible temperature by means of which the desired kappa reduction is effected, in order to obtain the highest possible viscosity.
 14. Method according to claim 12, characterized in that in order to determine the amount of alkali to be introduced into the oxygen stage, the amount of residual alkali passing into the oxygen stage is deducted from said alkali charge.
 15. Method according to claim 14, characterized in that said amount of residual alkali is determined at least from the washing liquid obtained as filtrate from the wash following the oxygen stage.
 16. Method according to claim 14, characterized in that said amount of residual alkali is determined from the pulp entering the oxygen stage prior to adding fresh alkali.
 17. Apparatus for treating pulp comprising an oxygen bleaching apparatus (10) comprising at least one reactor (12, 14), a washing device (34, 36) preceding and following said bleaching apparatus (10) and required devices (18, 16) for feeding chemicals into the pulp and feeding the pulp into said at least one reactor (12, 14) and further devices for leading washer filtrates at least partly countercurrently from one washing stage into another, characterized in that the apparatus further comprises devices for determining residual alkali either directly or indirectly both from the pulp being fed into the oxygen stage and from the washing liquid obtained as filtrate from the washer following the oxygen stage. 